Hyperthyroidism (too much thyroid hormone) is a common disease of older cats. Signs include weight loss, changes in appetite, vomiting, increased thirst and urination, hyperactivity, and diarrhea. High levels of thyroid hormone can cause heart disease. Methimazole is an anti-thyroid medication used to treat cats, and per a veterinarian’s order, the drug can be compounded into a transdermal gel for those cats that are difficult to “pill”. Transdermal gels can be applied to the inner ear and medication is absorbed across the skin. Simple precautions must be taken to avoid inadvertent absorption of the drug by the person administering the medication. Anti-thyroid drugs like methimazole do not offer a cure for hyperthyroidism but rather act to reduce thyroid hormone production and thus lessen its effects on various organ systems. Veterinarians at the School of Veterinary Medicine, Louisiana State University, treated hyperthyroid cats with transdermal methimazole and assessed serum thyroid concentrations and clinical response. Clinical improvement was observed, and thyroid hormone concentrations decreased significantly. No adverse effects were reported.